Heracleum mantegazzianum, commonly known as giant hogweed,[1] [2] [3] is a monocarpic perennial herbaceous plant in the carrot family Apiaceae. H. mantegazzianum is also known as cartwheel-flower,[2] [3] giant cow parsley,[4] giant cow parsnip,[5] or hogsbane. In New Zealand, it is also sometimes called wild parsnip (not to be confused with Pastinaca sativa) or wild rhubarb.[2]
Giant hogweed is native to the western Caucasus region of Eurasia. It was introduced to Britain as an ornamental plant in the 19th century, and has also spread to other areas in Western Europe, the United States, and Canada.[1] Its close relatives, Sosnowsky's hogweed and Persian hogweed, have similarly spread to other parts of Europe.
The sap of giant hogweed is phototoxic and causes phytophotodermatitis in humans, resulting in blisters and scars. These serious reactions are due to the furanocoumarin derivatives in the leaves, roots, stems, flowers, and seeds of the plant. Consequently, the jurisdictions of many regions consider it a noxious weed.[1]
The species name mantegazzianum refers to Paolo Mantegazza (1831–1910), Italian traveller and anthropologist.[6]
Giant hogweed typically grows to heights of NaNabbr=onNaNabbr=on.[1] Under ideal conditions, a plant can reach a height of 5.5m (18feet).[7] [8] The leaves are incised and deeply lobed. A mature plant has huge leaves, NaNabbr=onNaNabbr=on wide,[9] and a stout, bright green stem with extensive dark reddish-purple splotches and prominent coarse white hairs, especially at the base of the leaf stalk. Hollow, ridged stems are 3– in diameter, occasionally up to 10frac=2NaNfrac=2 in diameter, and can grow to more than 40NaN0 high. Dark red spots on the stem each surround a single hair.[10] The umbrella-shaped inflorescence, called a compound umbel, may be up to 100abbr=onNaNabbr=on in diameter across its flat top. The flowers are white or greenish white and may be radially symmetrical or strongly bilaterally symmetrical (zygomorphic).[8] The fruit is a schizocarp, producing seeds in dry, flattened, oval pairs.[8] The seeds are approximately 1frac=4NaNfrac=4 long, with a broadly rounded base and broad marginal ridges, coloured tan with brown lines (so-called oil tubes) running of the seed's length.
The life cycle of giant hogweed consists of four phases:[11]
During the first few years of growth, the leaves and stem of a pre-flowering plant die over the winter. In the spring, the plant grows back from its root. In other words, the giant hogweed is a herbaceous perennial.
A giant hogweed plant usually produces a flowering stalk in 3–5 years,[1] [12] but plants may take up to eight years to flower if conditions are unfavourable. In the Czech Republic, a single plant reached twelve years old before flowering.[13] In any case, when the plant finally flowers, it does so between June and July (in the northern hemisphere).
Seeds are typically produced in August. A single flowering plant will produce 20,000 seeds on average[14] with seed production varying between 10,000 and 50,000 seeds per plant.
Giant hogweed is a monocarpic perennial, that is, after a mature plant flowers and produces seed, the entire plant dies. During the following winter, tall dead stems mark the locations where the flowering plants once stood.
The seeds are dispersed short distances by wind, but can travel longer distances by water, animals, and people. Most seeds (95%) are found in the top 50NaN0 of the soil within a few metres of the parent plant. Seeds may stay alive in the seed bank for more than five years.
A seed deposited in the seed bank is initially dormant. Dormancy is broken by the cold and wet conditions of autumn and winter, and so freshly deposited seeds lie dormant until at least the following spring, at which time approximately 90% of the previously dormant seeds will germinate. The rest remain dormant in the seed bank.
Seeds normally result from cross-pollination between two or more plants but self-pollination is also possible. More than half the seeds produced by self-pollination will germinate and give rise to healthy seedlings. Hence a single isolated seed may give rise to a new colony of plants.
The various species of the genus Heracleum are similar in appearance, but vary in size. H. mantegazzianum is among the tallest, typically reaching 40NaN0 high (and sometimes more than 5m (16feet) high), whereas Heracleum species native to western Europe, such as H. sphondylium ([common] hogweed), or North America, such as H. maximum (cow parsnip), rarely exceed 30NaN0 high. There are considerable differences in the size of the umbel, leaves, and stem of H. mantegazzianum as well.
The following table compares H. mantegazzianum and H. maximum by feature:
| H. mantegazzianum (giant hogweed) | H. maximum (cow parsnip) | H. sphondylium ([common] hogweed) | ||
|---|---|---|---|---|
| Height | Typically 3to tall | Up to 2.5abbr=onNaNabbr=on tall | Up to 2abbr=onNaNabbr=on tall, rarely 2.5 m | |
| Leaves | Compound, lobed leaves typically 100abbr=onNaNabbr=on wide, up to 150abbr=onNaNabbr=on wide; mature leaf has deep incisions and serrated edges | Compound, lobed leaves up to 60abbr=onNaNabbr=on; mature leaf is less incised with less jagged edges | Compound, lobed leaves up to 60abbr=onNaNabbr=on; mature leaf is with rounded, less jagged edges, hairy | |
| Stem | Green stems from 3– in diameter, occasionally up to 10frac=2NaNfrac=2 in diameter, with dark reddish-purple splotches and coarse white hairs at the base of the leaf stalk | Green ridged stems up to 5frac=2NaNfrac=2 in diameter with fine white hairs (no purple splotches) | Grey-green ridged stems up to 3frac=2NaNfrac=2 in diameter, coarsely hairy (no purple splotches) | |
| Flowers | White umbel is typically 80abbr=onNaNabbr=on in diameter, up to 100abbr=onNaNabbr=on in diameter, with 50–150 flower rays per umbel; flowers from mid-June to mid-July | White umbel up to 300NaN0 in diameter with 15–30 flower rays per umbel; flowers from late May to late June | White (occasionally pinkish) umbel up to 250NaN0 in diameter with 12–25 flower rays per umbel; flowers from June to September (sometimes as early as April) | |
| Fruit | Narrower oval fruit 9–11 mm long | Heart-shaped fruit | Broad oval fruit 7–10 mm long |
Many more species exist; in Europe, over 20 species are found of the genus Heracleum alone. Few of these reach a similar size, and several are phototoxic to a lesser degree. Two other species, H. sosnowskyi and H. persicum, do reach similar sizes, and are equally noxious.
Giant hogweed is widespread throughout western and northern Europe, especially along terrains such as coastal areas and riverbanks. By forming dense stands, it can displace native plants and reduce wildlife habitats.[15] It has spread in the northeastern and northwestern United States and southern Canada, and is an invasive species across western Europe; in sites where it has settled, it overtakes the local native species, H. sphondylium.
In Canada, the plant occurs in most provinces, except in the prairies. It has been seen in Quebec since the early 1990s.[16] The plant's spread in Ontario began in the southwest and was seen in 2010 in the Greater Toronto Area and Renfrew County near Ottawa.[17]
In the US, giant hogweed occurs in Maine, Wisconsin, and south to Indiana, Michigan, Maryland, and New Jersey.[18] [19] In June 2018, it was reported growing in Virginia and North Carolina.[20] [21] The plant is federally listed as a noxious weed in the US.
The closely related hogweed Heracleum sosnowskyi is widespread in Russia and the Baltic states, and present in eastern Europe.
Heracleum mantegazzianum is native to the western Caucasus region of Eurasia.[1] Because of its impressive size, giant hogweed was brought to Europe and North America as an ornamental plant and garden curiosity.
The following historical information[22] [23] grew out of the European Giant Alien Project, which began in 2005.
Heracleum mantegazzianum was first described in scientific literature in 1895, but by that time more than a dozen European countries had already imported the plant as an "ornamental curiosity". The introduction of H. mantegazzianum was first recorded in Great Britain in 1817 when it was put on the seed list at the Kew Botanic Gardens in London. The first natural population was recorded in 1828, growing wild at Shelford in Cambridgeshire.[24]
The spread of H. mantegazzianum throughout Europe continued unabated until the middle of the 20th century, at which time the dangers of giant hogweed had become more widely known. Despite the warnings, however, the plant continued to be used by gardeners, beekeepers, and farmers (for cattle fodder) for another 50 years. Heracleum mantegazzianum was finally delisted by the Royal Horticultural Society of Great Britain in 2002.
During the 20th century, giant hogweed was transported to the US and Canada for display in arboreta and Victorian gardens. The earliest recorded planting in North America was in 1917, in gardens near Highland Park in the city of Rochester, New York.
By 1950, giant hogweed had appeared in southern Ontario, and within a quarter-century the plant was firmly established in Ontario. It was first collected from Nova Scotia in 1980 and Quebec in 1990. Giant hogweed was still available for sale in Canadian nurseries as late as 2005.
On the west coast of North America, H. mantegazzianum appeared in Oregon, Washington, and southwestern Canada, but it is not clear how the species found its way into this region. The first reports of giant hogweed in British Columbia were published in the 1930s.
Heracleum mantegazzianum has not spread in Russia. Another similar very large hogweed species, H. sosnowskyi, also native to the Caucasus region, was introduced into Russian agriculture starting from 1947 as a fodder plant and later spread extensively on its own throughout Russia and some other countries of eastern Europe.
Because of its phototoxicity and invasive nature, giant hogweed is often actively removed. The European Union funded the Giant Alien project to combat the plant.[25] [26] [27] On 2 August 2017, it added the species to its List of Invasive Alien Species of Union concern,[28] thus placing restrictions on keeping, importing, selling, breeding and growing it, and requiring governments to detect and eradicate it throughout the EU. In the United Kingdom, the Wildlife and Countryside Act 1981 makes it an offence to plant or cause giant hogweed to grow in the wild.[29]
In the US, hogweed is regulated as a federal noxious weed, and is illegal to import into the US or move interstate without a permit from the Department of Agriculture (USDA).[30] The USDA Forest Service states that pigs and cattle can eat it without apparent harm.[31] The New York State Department of Environmental Conservation has had an active program to control giant hogweed since 2008.[32] In 2011, Maine state horticulturists reported that it has been found at 21 different locations in Maine, with the number of plants ranging from one to a hundred.[33]
See also: Phototoxicity. The sap of the giant hogweed plant is phototoxic. The sap contains furanocoumarins, which leads to phytophotodermatitis, a serious skin inflammation.[34] A phototoxic reaction can begin as soon as 15 minutes after contact with the sap. Photosensitivity peaks between 30 minutes and two hours after contact but can last for several days. Authorities advise that all humans (especially children) should stay away from giant hogweed.[35] [36] [37] Protective clothing, including eye protection, should be worn when handling the plant. Parts of the body that come into contact with the sap of giant hogweed should be immediately washed with soap and cold water, and further exposure to sunlight should be avoided for at least 48 hours.[15] Other Heracleum species such as H. maximum, are likewise phototoxic, and hence similar caution is advised. Owing to physical similarities to other hogweeds in the genus Heracleum, giant hogweed and its close relatives are sometimes mistaken for harmless plants.[38]
Discounting phototoxicity from contact with external tissues, poisoning via ingestion by humans is not known, and consumption of small amounts of the plant is not deemed to be harmful,[39] although should still be avoided due to the obvious extreme hazards created via proximity to the plant. Consumption by some other grazing animals does not appear to cause them harm, and sheep have been trialed as a giant hogweed control agent in Scotland.[40]
The 1971 album Nursery Cryme by the progressive rock group Genesis contains the song "The Return of the Giant Hogweed". The darkly humorous lyrics[41] describe an attack on the human race by the hogweed, long after the plant was first 'captured' and brought to England by a Victorian explorer.[42]
In her 1985 novel Curse of the Giant Hogweed, popular mystery author Charlotte MacLeod places her established character Peter Shandy and his colleagues in a fantasy version of Wales to investigate giant hogweed endangering Britain's hedgerows.[43]
The plant is featured in season 10, episode 3, of the AMC television series The Walking Dead, growing from zombies which a character encounters, rendering him unable to see properly.[44]